1. Field of the Invention
This invention relates to a semiconductor display device and to a method of driving the semiconductor display device. More particularly, the invention relates to an active matrix-type semiconductor display device having thin-film transistors (TFTs) fabricated on an insulating substrate, and a method of driving the active matrix-type semiconductor display device. In particular, the invention relates to an active matrix-type liquid crystal display device among the active matrix-type semiconductor display devices and to a method of driving the active matrix-type liquid crystal display device.
2. Description of the Related Art
In recent years, technology has been rapidly developed for fabricating TFTs by forming a semiconductor thin film over a cheaply available glass substrate. The reason is due to an increased demand for the active matrix-type liquid crystal display devices (liquid crystal panels).
An active matrix-type liquid crystal display device is the one in which pixel TFTs are arranged in several tens of thousands to several millions of pixel regions arranged like a matrix (this circuit is called active matrix circuit), and the electric charges going into, and coming out from, the pixel electrodes of the pixel regions are controlled by a switching function of pixel TFTs.
The active matrix circuit has heretofore been employing TFTs of amorphous silicon formed over a glass substrate.
In recent years, there has been realized an active matrix-type liquid crystal display device having TFTs using a polycrystalline silicon film formed on a quartz substrate. In this case, a peripheral drive circuit for driving the pixel TFTs can be fabricated over the same substrate as the active matrix circuit.
There has also been known technology for fabricating TFTs by forming a polycrystalline silicon film over a glass substrate by utilizing such technology as laser annealing. This technology makes it possible to form the active matrix circuit and the peripheral drive circuit in an integrated manner over the same glass substrate.
In recent years, the active matrix-type liquid crystal display device has frequently been used as a display of personal computers. The active matrix-type liquid crystal display device of a large screen has been used for desktop personal computers, too, in addition to notebook personal computers.
Attention has also been given to a projector using a small active matrix-type liquid crystal display device which features sharp image, high resolution and high image quality. Particularly, a projector for high vision capable of displaying image maintaining a higher resolution is drawing attention.
Here, the liquid crystal display device must execute an inverse drive to prevent the liquid crystal elements from being deteriorated. Concretely speaking, as shown in FIG. 3A, a video signal is inverted from positive to negative after every frame period with a potential of an opposing electrode (hereinafter referred to as opposing common potential, VCOM) as a center potential (constant value). Usually, in this case, a source signal line drive circuit is driven with a voltage having an amplitude slightly broader than the amplitude of the video signal in order to reliably write the video signals into the source signal line. This is because the analog switch has been constituted by a pair of N-channel TFT and P-channel TFT, a current at the time of writing the signal must be large enough to reliably write the signal into the source signal line, and the switch must be reliably turned off to prevent the leakage of electric charge once written into the source signal line from the analog switch. Usually, the ON/OFF margin of the analog switch is about 3 [V] by taking a threshold value +α of the TFTs into consideration. Concretely speaking, when the amplitude of the video signal written into the source signal line is ±5 [V], the amplitude of the drive voltage of the source signal line drive circuit (analog switch) becomes ±8 [V]. A gate signal line drive circuit, too, is driven with an amplitude of ±8 [V] in order to maintain a voltage across gate and source of the pixel TFT by taking the threshold value into consideration.
Here, if attention is given to the electric power consumed in driving the liquid crystal display device, the buffer unit of the source signal line drive circuit consumes a large proportion of electric power among the electric power consumed by the whole display device. Therefore, if the consumption of electric power could be decreased by lowering the drive voltage of the source signal line drive circuit, then, the consumption of electric power by the whole display device can be greatly decreased.
According to the above inverse drive system, for example, the drive voltage is ±8 [V](16 [V]) when VCOM is 0 [V] constant and the amplitude of the video signal is from −5 to 5 [V](10 [V]) by taking the ON/OFF margin (3 [V]) of the analog switch into consideration.
Considered below is a method of inverting VCOM from positive to negative relative to a video signal that is inverted from positive to negative for every frame period. Referring to FIG. 3B, the video signal is 2.5 [V] in a given frame, the opposing VCOM is −2.5 [V] in a given frame and in a next frame, the video signal is −2.5 [V] and the opposing VCOM is 2.5 [V]. In each frame, the voltage applied to the liquid crystal element is 5 [V], i.e., a potential difference between the video signal and VCOM is 5 [V] like in an ordinary case, though the video signal has an amplitude of from −2.5 to 2.5 [V](5 [V]). When the ON/OFF margin of the analog switch is considered to be 3 [V] like in the above case, therefore, the drive voltage becomes ±5.5 [V](11 [V]), and the consumption of electric power can be decreased by about 47[%].
Further, in the source signal line drive circuit, in general, the TFT must have a large current ability since the source signal line has a large capacitive load and the drive frequency is high. Accordingly, the TFTs constituting the source signal line drive circuits, usually, have a small gate width (L) and a large channel length (W). Therefore, these TFTs are likely to be more deteriorated than other TFTs. A decrease in the buffer voltage of the source signal line drive circuits by 5 [V] is equal to improving the reliability of TFTs in the source signal line drive circuits.
On the other hand, the opposing common inverse drive causes an increase in the burden on the gate signal line drive circuits and on the pixel TFTs. In the pixel portion, the opposing electrode and the source region of the pixel TFT (in the pixel TFT, hereinafter, the region on the side connected to the source signal line is defined as the drain region and the region on the side connected to the liquid crystal element is defined as the source region, this positional relationship is maintained even when the potential of the video signal is inverted) are coupled together through capacity with a liquid crystal element sandwiched therebetween. When this capacity is dominating compared to other capacities in the drive circuit unit, a change in the VCOM in a state where the pixel TFT is off is accompanied by an equal amount of change in the potential in the source region of the pixel TFT in order to preserve the potential difference across the electrodes of the capacity. Concretely speaking, when a voltage applied to the liquid crystal element is from −5 to 5 [V] while VCOM VCOM=−2.5 [V], the potential in the source region of the pixel TFT could become from −7.5 to 2.5 [V]. When the voltage applied to the liquid crystal element is from −5 to 5 [V] while VCOM=2.5 [V], the potential in the source region of the pixel TFT could become from −2.5 to 7.5 [V] (FIGS. 3C and 3D).
When the drive voltage amplitude of the gate signal line drive circuit is ±8 [V] in this state, the ON/OFF margin of the pixel TFT becomes 0.5 [V], and normal operation is not often accomplished depending upon the threshold value of the pixel TFT. To maintain a margin of 3 [V] like in the source signal line drive circuit, the amplitude of the drive voltage of the gate signal line drive circuit must be ±10.5 [V] like in FIG. 3E.
Thus, the voltage increases across gate and source of the pixel TFT. Reference is now made to FIG. 4A. When VCOM has an amplitude of ±2.5 [V], the potential in the source region of the pixel TFT could become from −7.5 to 7.5 [V]. At this moment, the potential which the gate electrode could assume is ±10.5 [V]. It is therefore considered that the voltage across gate and source of the pixel TFT is from −18 to +18 [V].
FIG. 5 illustrates voltage-current characteristics of an N-channel TFT, wherein the abscissa represents a voltage (VGS) across gate and source and the ordinate represents a drain current (ID) When a large inverse bias voltage (a voltage of the gate electrode of a potential lower than the potential of the source region) is applied to the gate electrode, the drain current often increases suddenly. That is, when the voltage across gate and source is −18 [V] in the pixel TFT, the stored electric charge leaks through the pixel TFT that has been turned off. Besides, when such a large voltage is applied across gate and source, a problem arouses concerning the gate breakdown voltage. Because of this problem, the opposed common inverse drive system has not been almost put into practice and, instead, the ON/OFF margin of the pixel TFT is cut and VCOM is permitted to possess only a small degree of amplitude.